I thoroughly enjoyed The Burning Crusade. It was the first time my guild got into raiding, and though I don't enjoy raiding on a consistent basis, Karazhan was a blast! After that, Wrath was good. I became slightly burnt out towards the end of that expansion and when news of Cataclysm dropped I couldn't wait to get back in.
I will defend Cata's launch since as a Vanilla lover I enjoy Azeoth. Redoing parts of the map and redoing some of the beginning quests and hubs was a breath of fresh air (except the new undead town designs, have never liked them). Everything after the Cata launch I was not impressed with, though I did continue to play up until a few months ago. But now that I am here, I have somewhere to go when expansions continue to blow.
There have been so many changes I haven't enjoyed, personally; LFR, LFG (but I was in a REALLY great guild so maybe I am biased on these), ability pruning, talent tree shrinking, Cross realm anything, fast leveling, level boost for fast access to new content, hero classes (these should start from level 1 like all the others, as it would allow the player to immerse themselves much better), etc.... But overall, I will remain a player as the lore and stories are what I fell in love with way back in Warcraft Orcs and Humans.
So in summary:
The Burning Crusade > Wrath of the Lich King > Cataclysm (launch) > Mists of Pandaria > Cataclysm (rest of) > (Space reserved for Legion, it can't possibly be worse then WoD.....right?) > Warlords of Draenor